1. Field of the Invention
The present invention relates to a composite dielectric molded product, and particularly to a composite dielectric molded product and a lens antenna using the same.
2. Description of the Related Art
In recent years, intelligent transport systems (ITS) for the next generation have been actively developed, and functions for supporting safe cruise driving have been increasingly developed. Particularly, an external environmental detection system of the ITS, functioning as the eye of an automobile, has been considered as most important, and detection systems using infrared rays, CCD or the like, have been developed. However, these detection systems have the problem of failing in the rain, and increasing the cost.
Therefore, a radar utilizing a millimeter wave (76 GHz) is considered to be used as external environmental detection means. Examples of such a milli-wave antenna include a planar antenna having a planar outgoing plane, a lens antenna having a convexly curved outgoing plane, and the like. The lens antenna is considered particularly excellent in antenna efficiency and detection angle.
Such a lens antenna generally comprises a lens body having a convex outgoing plane and a primary transmitter provided behind the lens body. Particularly, a composite dielectric material comprising a resin and a dielectric inorganic filler exhibiting a high dielectric constant even with a small thickness and excellent productivity is used as the material for the lens body for an on-vehicle lens antenna in which the thickness of the lens body must be decreased. The lens body is generally molded by injection molding from the viewpoint of molding cost and molding precision.
However, the values of antenna gain and side lobe of a lens antenna in a lens body (composite dielectric molded product) obtained by molding a conventional composite dielectric material cannot be achieved according to design, and variation occurs in characteristics to deteriorate yield.
Accordingly, it is an object of the present invention to provide a composite dielectric molded product exhibiting excellent properties such as antenna gain, side lobe, etc. when used for a lens antenna, and exhibiting less variation of properties in one individual product and between individual products.
In order to achieve the object of the present invention, a composite dielectric molded product in accordance with a first aspect, comprises a composite dielectric material containing a dielectric inorganic filler and an organic polymer material, wherein the dielectric constant anisotropy is in the range of about 1.00 to 1.05. The dielectric constant anisotropy represents the ratio (A/B) of dielectric constant A in the direction in which the dielectric constant is maximum to dielectric constant B in the direction in which the dielectric constant is minimum.
By using the composite dielectric material and controlling the dielectric constant anisotropy of the obtained molded product, a composite dielectric molded product having excellent electric properties and less variation in properties can be obtained. With attention to the fact that the dielectric constant of a composite dielectric molded product varies with the direction of an electric field depending upon the composite dielectric material used and molding conditions, the inventors found that the composite dielectric molded product exhibiting large variation in the dielectric constant produced an electric field direction in which the desired dielectric constant property cannot be obtained and in which there was variation in properties of the composite dielectric molded product. Therefore, it was found that by decreasing the variation in the dielectric constant with respect to the electric field direction, i.e., by controlling dielectric constant anisotropy to about 1.00 to 1.05, the above-described problem could be resolved, leading to the achievement of the present invention.
In a composite dielectric molded product according to a second aspect of the present invention, the composite dielectric material preferably has a melt viscosity of about 170 Paxc2x7s or more at a shear rate of 1000 Sxe2x88x921 during molding. With such a melt viscosity, the dielectric constant anisotropy can be controlled in the range of about 1.00 to 1.05 even in an injection molding method in which the dielectric constant anisotropy of the composite dielectric molded product is liable to increase.
In a composite dielectric molded product according to a third aspect of the present invention, the organic polymer material preferably comprises a thermoplastic resin. By using this organic polymer material, the composite dielectric material can be molded by injection molding, thereby decreasing production cost and permitting easy molding with high precision of form.
In a composite dielectric molded product according to a fourth aspect of the present invention, the organic polymer material preferably comprises a thermoplastic resin containing a resin filler. By using such an organic polymer material, the dielectric constant anisotropy can be decreased because the orientation of the dielectric inorganic filler is suppressed by the resin filler.
In a composite dielectric molded product according to a fifth aspect of the present invention, the dielectric inorganic filler preferably comprises at least one oxide, carbonate, phosphate or silicate of IIa, IVa, IIIb or IVb group elements, and compound oxides containing IIa, IVa, IIIb or IVb group elements. By using such a dielectric inorganic filler, a high dielectric constant can be obtained even when the composite dielectric molded product has a small thickness.
A lens antenna according to a sixth aspect of the present invention comprises at least a lens unit having a convex outgoing plane and a primary transmitter provided behind the lens unit, wherein the lens unit comprises a composite dielectric molded product in accordance with any one of the first to fourth aspects of the present invention. In the lens antenna having this construction, the antenna gain can be increased, and the side lobe and variation in properties can be decreased.
In a lens antenna according to a seventh aspect of the present invention, the lens unit preferably comprises a lens body and a matching layer formed on the surface of the lens body, for matching the lens body with the air. By providing the matching layer on the lens body, reflection of electromagnetic waves can be further suppressed during emission and reception of electromagnetic waves.